1. Field of the Invention
This invention relates to a diversity antenna system which includes two planar inverted F antennas which have a small common ground plane. Four embodiments of the invention are disclosed herein.
2. Description of the Related Art
In its simplest form, the diversity technique, as it applies to antennas for RF data and wireless communication devices, provides a means of achieving reliable and enhanced system performance through the use of an additional antenna. A diversity antenna system utilizes two antennas which sample the RF signal to determine the strongest signal to enable the communication device to utilize the strongest RF signal. To meet the requirement of sustained and fast rate of data transfer, specific emphasis has been recently placed on diversity antennas in RF data communication. Despite the enhanced reliability and the improved performance of an antenna system with the diversity scheme, its adoption to a compact wireless system is not widespread. Theoretically, the spatial diversity technique requires a physical separation of one wavelength between the two antennas. In many practical applications, it may not be feasible to provide the required separation between the two antennas of a spatial diversity scheme. The requirement of a wide separation between the two antennas of a diversity scheme also requires a longer feed cable to the individual antennas from a common RF source point. The resulting longer feed cable leads to the problem of ensuring effective shielding of the cable, the consequent RF power loss in the cable and the undesirable interference effect on system performance particularly at a higher frequency band. The above-mentioned shortcomings apply to diversity schemes consisting of conventional external antennas which have been in existence for a long time as well as with the recently evolving internal antenna. In view of the above constraints associated with the conventional diversity scheme, emphasis is being shifted to arrive at a compactness of the overall spatial diversity scheme which meets acceptable performance standards.
Of late there has been an increasing emphasis on internal antennas instead of a conventional external wire antenna. The concept of internal antenna stems from the avoidance of a protruding external radiating element by the integration of the antenna into the device itself. Internal antennas have several advantageous features such as being less prone for external damage, a reduction in overall size of the handset with optimization, and easy portability. The printed circuit board of the communication device serves as the ground plane of the internal antenna. Among the various choices for internal antennas, the PIFA appears to have great promise. The PIFA is characterized by many distinguishing properties such as relative lightweight, ease of adaptation and integration into the device chassis, moderate range of bandwidth, Omni directional radiation patterns in orthogonal principal planes for vertical polarization, versatility for optimization, and multiple potential approaches for size reduction. Its sensitivity to both vertical and horizontal polarization is of immense practical importance in mobile cellular/RF data communication applications because of the absence of the fixed antenna orientation as well as the multi-path propagation conditions. All these features render the PIFA to be a good choice as an internal antenna for mobile cellular/RF data communication applications.
The PIFA also finds useful applications in diversity schemes. Despite all of the desirable properties of a PIFA, the PIFA has the limitation of a rather large physical size for practical application. A conventional PIFA should have the semi-perimeter (sum of the length and the width) of its radiating element equal to one-quarter of a wavelength at the desired frequency. With the rapidly advancing size miniaturization of the radio communication devices, the space requirement of a conventional PIFA is a severe limitation for its practical utility. Further, the internal antenna technology is relatively new and is in an evolving stage of development. The combination of inherent shortcomings associated with the size of the PIFA and the requirement of even larger space or volume for multiple PIFAs seems to be the primary reason for the non-feasibility of the use of PIFA for diversity schemes of modern wireless communication systems.
To assist in the understanding of a conventional PIFA, a conventional single band PIFA assembly is illustrated in FIGS. 9A and 9B. The PIFA 110 shown in FIG. 9A and FIG. 9B consists of a radiating element 101, a ground plane 102, a connector feed pin 104a, and a conductive post or pin 107. A power feed hole 103 is located corresponding to the radiating element 101. The connector feed pin 104a serves as a feed path for radio frequency (RF) power to the radiating element 101. The connector feed pin 104a is inserted through the feed hole 103 from the bottom surface of the ground plane 102. The connector feed pin 104a is electrically insulated from the ground plane 102 where the pin passes through the hole in the ground plane 102. The connector feed pin 104a is electrically connected to the radiating element 101 at 105a with solder and the body of the feed connector 104b is electrically connected to the ground plane at 105b with solder. The connector feed pin 104a is electrically insulated from the body of the feed connector 104b. A through hole 106 is located corresponding to the radiating element 101, with the conductive post or pin 107 being inserted through the hole 106. The conductive post 107 serves as a short circuit between the radiating element 101 and the ground plane 102, The conductive post 107 is electrically connected to the radiating element 101 at 108a with solder. The conductive post 107 is also electrically connected to the ground plane 102 at 108b with solder. The resonant frequency of the PIFA 110 is determined by the length (L) and width (W) of the radiating element 101 and is slightly affected by the locations of the feed pin 104a and the shorting pin 107. The impedance match of the PIFA 110 is achieved by adjusting the diameter of the connector feed pin 104a, by adjusting the diameter of the conductive shorting post 107, and by adjusting the separation distance between the connector feed pin 104a and the conductive shorting post 107.
In this invention, several new embodiments of compact diversity PIFAs having a small and common ground plane are disclosed. This invention demonstrates that it is possible to retain the performance of individual antennas of a spatial diversity antenna scheme even when the separation between the antennas is only a fraction of a wavelength. In the first embodiment of this invention, two PIFAs are placed back to back on a small rectangular ground plane. The two PIFAs are placed such that the shorted ends of the PIFAs face each other. Such an arrangement ensures better isolation between the two PIFAs despite being placed in close proximity to one another. In the second embodiment of this invention, the ground plane is bent at its opposite ends to form vertical sections. The two PIFAs are placed (outward) on the vertical sections at the opposite ends of the ground plane. Such an arrangement of PIFAs allows the placement of some system components between the two vertical sections of the bent ground plane. The distortion of the radiation patterns of the PIFAs is also minimized despite the presence of some components between the two PIFAs. This is mainly due to the blockage effect offered by the vertical sections of the ground plane. With a significantly different design configuration, in the third embodiment of this invention, there is no physical separation between the two PIFAs placed on a common rectangular ground plane. Only a single shorting pin or post partitions the two diversity PIFAs resulting in an extremely simple and compact diversity PIFA. The virtual electrical partitioning between the two radiating elements is realized through the common shorting post. The virtual electrical partitioning between the two radiating elements in lieu of the proposed choice of placement of the shorting post overcomes the need for physical separation between the two radiating elements to serve as separate antennas of a diversity scheme. In the fourth embodiment, which is a modification of th e third embodiment, the two PIFAs, which are not physically separated, are placed on a common L-shaped ground plane. The partitioning of the two antennas is again realized through a common shorting post. Unlike the third embodiment, the two PIFAs of the fourth embodiment are oriented orthogonal to each other. The basic concepts proposed in all the embodiments of this invention have been proved through the design of diversity PIFAs for ISM Band applications. In all of the above-described embodiments, good VSWR performance is achieved. The individual PIFAs of the embodiments show satisfactory gain performance. The invention disclosed herein can be extended to other frequency bands of interest.
One of the principal objects of the invention is to circumvent the requirement of wide separation between the two internal PIFAs of a spatial diversity scheme.
A further object of the invention is to provide an efficient design of a diversity antenna utilizing only a small ground plane that is common for both the antennas.
Still another object of the invention is to provide a compact diversity PIFA characterized with the salient feature of the absence of physical partitioning between the two antennas.
Yet another object of the invention is to utilize the common ground plane of non-rectangular shapes in diversity PIFAs.
Another object of the invention is to design individual PIFAs of a diversity antenna which are compact in size.
Still another object of the invention is to provide diversity PIFAs having the desirable features of configuration simplicity, compact size, cost effective to manufacture and ease of fabrication.
These and other objects will be apparent to those skilled in the art.